Declaring Classes and Sending Messages in Objective-C

Object-oriented programming languages enable you to declare classes, create derived classes (subclass), and send messages to the objects instantiated from a class. This is the essence of object-oriented programming and part of the object-oriented extensions that Objective-C adds to C. To ensure that everything operates smoothly, compiler directives are available that enable you to inform the compiler of your classes by using @class and #import.